The present invention relates to a process for fabricating a hybrid substrate that can be used in the field of optics, electronics or optoelectronics. The terms “optics”, “electronics” and “optoelectronics” in general include microelectronics, nano-electronics, micro-optoelectronics, nano-optoelectronics and components technology.
A hybrid substrate is a substrate comprising at least two layers of material of the same or different nature, the term “nature” covering both the chemical nature of the material and its physicochemical properties and/or its crystalline orientation. Among these hybrid substrates are in particular those known by the acronym “SeOI”, which stands for “Semiconductor On Insulator”. Such a substrate comprises an insulator layer, for example made of oxide, buried between a generally thin layer of semiconductor material, the “active” layer, and a bulk substrate or “receiver” substrate made of semiconductor material.
In the rest of the description and the claims, the term “insulator” denotes an electrically insulating material, possibly having a high dielectric permittivity. SeOI substrates are fabricated, for example, by a process known by the trademark SMART-CUT® which typically comprises the following steps:
formation or deposition of an insulator layer on a first substrate, called a “donor” substrate, so that an interface called a “bonding” interface exists between them;
implantation of atomic species into the donor substrate so as to form a zone of weakness therein;
bonding a second substrate, called a “receiver substrate”, onto the free surface of the insulator by molecular adhesion; and
detachment of the rear part of the donor substrate along the zone of weakness.
In the field of microelectronics, the surface quality of the active layer of semiconductor material is of very great importance. More precisely, the roughness and the absence of surface defects on this active layer are parameters that have to be optimized so that the future components that will be produced from these SeOI substrates will be of optimum quality. Now, various types of surface defects may appear after the active layer has been transferred onto the receiver substrate. These defects are in particular:
non-transferred zones (known as NTZs);
blisters;
voids; and
crystal-oriented voids or COVs.
All these defects are due to poor transfer, to the presence of underlying defects in the various layers of the hybrid substrate, to the quality of the bonding at the interface or, very simply, to the processes used to fabricate such substrates, such as, for example, the implantation of atomic species and the heat treatment. The defects present at the bonding interfaces will become sites for gas trapping during the various steps of the process and will thus swell and form voids or COVs. Thus, to give an example in the case of a hybrid substrate comprising, in succession, a silicon support substrate, covered with a thermal oxide layer, then with a tetraethylorthosilicate (TEOS) oxide layer obtained by low-pressure chemical vapor deposition (LPCVD) deposition and, finally, with a germanium active layer, the gaseous elements may have several origins. As a reminder, the term “LPCVD TEOS” will denote a silicon oxide (SiO2) obtained from a TEOS precursor by a LPCVD technique.
The aforementioned gaseous elements may derive in particular:
from the hydrogen or helium supplied during the atomic species implantation step for the purpose of forming the zone of weakness, the quantity of these gaseous elements depending on the type of implanter used and on the implantation conditions (dose and energy);
from the desorption of water (H2O) molecules present at the bonding interface between thermal oxide and the TEOS oxide; and
from the TEOS oxide if the densification of the latter has not been sufficient, because of the diffusion of carbon compounds.
In addition, it should be noted that the smaller the thickness of the active layer, the larger the number of defects. This is because when the active layer is thick enough, defects of the blister or void type are generally retained within its thickness and consequently appear less on its surface. FIG. 4 herein provides an illustrative example of these defect problems. This is a graph representing the concentration C of H+ ions per cm2 as a function of the depth P expressed in nanometers in a particular hybrid substrate, the results having been obtained by secondary ion mass spectroscopy (SIMS).
More precisely, this hybrid substrate is the result of bonding between a silicon (Si) support substrate that has undergone a thermal oxidation and a germanium (Ge) donor substrate on which a layer of silicon oxide (SiO2) has been deposited. The bonding interface is therefore between two oxides, one belonging to the support substrate and the other belonging to the donor substrate. In the graph of FIG. 4, the bonding interface between the two SiO2 layers is located at −200 nm. The support substrate has not been shown in FIG. 4—only its linking interface with the SiO2 layer is shown, and this interface is located at −400 nm.
Curve a shown as a solid line represents the results obtained in the germanium donor substrate covered with SiO2 before the two substrates are bonded together and before the implantation of atomic species using the SMART-CUT® process for the purpose of forming the zone of weakness within the germanium layer. The H+ ions lie mainly at the bonding interface between the SiO2 first layer and the germanium layer. Curve c shown as a bold line represents the results obtained in the same substrate after the implantation for forming the zone of weakness and before the bonding to the SiO2 second layer. It should be noted that the vertical line at −200 nm corresponds to an artifact. The values start only at about −200 nm as this corresponds to the implantation carried out before the bonding of the SiO2 second layer. In the particular case of germanium, detachment does not take place in the region of maximum implantation, but just a little after (about 550 nm), which will explain the appearance of curve b below.
Curve b shown as a dotted line represents the results obtained after the bonding of the two SiO2 layers and after detachment and transfer of the germanium active layer. The distribution of the hydrogen species shows a build-up at the SiO2/Ge and SiO2/support substrate bonding interface. A small peak appears at −200 nm in the SiO2 layer. This corresponds to the bonding interface between the two SiO2 layers.
A substantial increase in the quantity of gas is observed in the insulator (SiO2) layer after the step of detaching and transferring the germanium active layer. This increase is the cause of the defects that are visible, after transfer, on the upper face of the transferred germanium layer.
In addition, in the field of semiconductors, the trend is toward ever greater miniaturization of components and reduction in their energy consumption. This supposes that the thickness of both the active layer and the buried oxide layer are reduced simultaneously. Now, reducing the thickness of these two layers also results in the appearance of defects, especially when the oxide layer has a thickness of less than 25 nm (25 nanometers) and the active layer has a thickness of less than 400 nm (400 nanometers).
Finally, among hybrid substrates are also those known by those skilled in the art as Direct Silicon Bonding (DSB) substrates. Such substrates comprise an active layer of semiconductor material directly bonded to a receiver substrate or bulk substrate, also made of semiconductor material, without the formation of an intermediate layer, especially without the formation of a buried oxide layer. The fabrication processes known at the present time for this type of DSB substrate also result in the appearance of defects when the active layer is thin.
To solve the aforementioned problems of the appearance of surface defects, the process described in US patent application no. 2002/0190269 is already known. The purpose of this process is to fabricate a hybrid substrate that comprises a germanium layer on silicon, while reducing the formation of bubbles at the bonding interface. More precisely, this substrate is obtained by implantation of hydrogen into a germanium donor substrate so as to form a zone of weakness therein, then by bonding it to a silicon receiver substrate, and finally by heat treatment to detach the rear part of the germanium substrate.
In one particular embodiment of that application, an “antibubble” layer of amorphous silicon is placed on the germanium substrate before bonding, so as to make the bonding interface hydrophilic and thus to reduce the formation of hydrogen bubbles when the germanium substrate is bonded to the silicon substrate. According to another embodiment, it is suggested to improve the roughness of the transferred germanium layer by depositing thereon a germanium buffer layer, formed by epitaxy. The purpose of these two solutions is to improve the quality of the bonding interface between the two substrates, but they require the addition of an additional layer, thereby complicating the process. Thus, improvements in these areas remain needed.